This is correct. There is only one clock source active, and it provides timing for all of the T1 interfaces. Your unit is configured to use t1 0/4 as the primary timing source, and if there is no clock available there then the unit will fall back to its internal clock.

You can use the service provider as both the primary and secondary clock source if more than one interface is connected them.

For example, I have a Multilink PPP service providing a PRI handoff. I'm using this configuration:

timing-source t1 0/1

timing-source t1 0/2 secondary

This way I still receive clock from the service provider if either T1 is down, and the service provider's clock will provide the timing for the t1 0/3 and t1 0/4 used for the PRI handoff.

If both service provider T1 go down, the until will still fall back to its internal clock for the PRI interfaces.

how to check clock source for each T1 in an adtran 908e with multiple T1's

OK, to expand on this a bit more....

General rules for T1s:

An Adtran TA900 series unit can only have one authoritative clock source for all of the T1s connected to it.

The TA900 will use its configured authoritative clock as a source for all connected T1s as well as its internal DSPs.

Every T1 link should have exactly one clock source. Not zero and not two.

So, in your example, the TA900 is receiving clock from T1 0/4. It will use that clock to synchronize its own internal electronics. It will also use that clock as a source for any other connected T1s.

As a rule, you want to receive clock (set as timing-source) from a T-1 that is connected to an upstream carrier (such as an ISP or a telco PRI). The TA900 will regenerate that clock out all other T1s. So if you have a telco PRI on T1 0/4, you're doing the right thing. If you have a customer PBX on T1 0/1, 0/2, or 0/3 you will want to configure that PBX to receive its clock from the T1 connected to the TA900. Timing will flow Carrier -> TA900 -> PBX. If you lose connection to the carrier, the TA900 will continue to clock the PBX from its internal oscillator.

If this doesn't make sense, reply and describe all of the connected T1 interfaces and to what they are connected.